Japanese encephalitis virus is the causative agent of an acute inflammatory illness of the central nervous system found throughout eastern Asia. A formalin inactivated, mouse-brain derived Japanese encephalitis virus vaccine has been developed. A report of severe urticarial reactions associated with immunization with Japanese encephalitis virus vaccine was published in 1991 in The Lancet. The rate of severe side-effects was reported to be 0.04%. The description of the adverse reactions was similar to allergic reactions seen when booster doses of rabies vaccine were given. The Centers for Disease Control were invited to participate in an investigation of the reactors. A case-control study found that persons developing these reactions after JE vaccination were more likely to have had a past history of urticaria (after hymenoptera envenomation, drugs, physical or other provocations or of idiopathic cause). Sixteen reactors were skin tested with the vaccine causing the allergic reaction at least 5 weeks after the reaction. None of the reactors showed skin test positivity. Acute and convalescent sera were drawn for testing for IgE levels and reactivity with the vaccine. These tests are still in progress. The mechanism and cause of these reactions is still unknown. The vaccine labeling will include adequate warnings to inform physicians and recipients of the vaccine about the allergic reactions.